1. Field of the Invention
This invention relates to a surface treatment method for treating the surface of an aluminum alloy containing, e.g., 2.0% (by weight; the same applies hereinafter) or more of Mg and having been processed at a high temperature, to thereby obtain surface-treated aluminum alloy high-temperature processed articles as exemplified by construction materials, transport machinery materials, ornamentals, vessels and so forth having a good anticorrosion, a good adhesion of coatings and a beautiful surface appearance.
2. Description of Related Art
Aluminum high-temperature processed articles as exemplified by superplastic processed articles have been not feasible for uses that require beauty, adhesion of coatings and anticorrosion, for the following reasons.
In the case of processed articles of superplastic materials whose surfaces have been clad with pure aluminum as in SUPRAL100 (trade name; available from Alcan Co.), there are some examples in which the processed articles are used after surface treatment with relatively small problem. However, since they are clad materials, a cost increase can not be avoided.
In general, in such instances, it can be contemplated that adhesion of coatings and anticorrosion can be improved by mechanically removing surface layers through, e.g., air blast using alundum or the like as an abrasive. This method, however, tends to cause warpage and strain, and, because of the abrasive that may be buried in the surfaces, may cause blackening, poor adhesion of coatings and so forth after further surface treatment such as coating.
It can be also contemplated to use the known melons as disclosed in the following publications. There, however, are problems also set forth below together.
Japanese Patent Laid-open No. 50-86540:
This discloses that an aluminum substrate is treated with an aqueous alkali silicate solution, and thereafter treated with a vinylidene chloride resin to form an anticorrosive coating.
This method has the problem that the treatment tends to become uneven when the treatment with an alkali silicate is applied, making it difficult to obtain homogeneous coatings.
Japanese Patent Laid-open No. 4-04781:
This discloses that an aluminum sheet surface is degreased with a weak alkali and thereafter treated in an aqueous solution containing an oxidizing agent to form a boehmite costing.
This method has the problem that defective coatings and blackening may occur during boehmite treatment and coating treatment.
Japanese Patent Laid-open No. 1-212775:
This discloses that etching is carried out in an aqueous acidic solution of pH 2 or leas containing chloride ions, followed by etching in an aqueous alkaline solution of pH 13 or more.
This method has the problem that it requires two-stage treatment, which makes productivity poor to cause a cost increase. Moreover, because of occurrence of smuts, difficulties such as defective coatings may be caused in the subsequent coating and chemical conversion.
Japanese Patent Laid-open No. 2-97700:
This discloses that degreasing is applied in an aqueous solution to which a surface active agent has been added, and thereafter anodic polarization electrolysis and cathodic polarization electrolysis are alternately carried out.
This method has the problem that an uneveness may be caused during electrolysis in the aqueous surface active agent solution, and, in an attempt to obtain uniform treated surfaces, electric power is required in a large quantity, leading to a cost increase. In addition, this method can not be applied to articles having complicated shapes.
On the surface of aluminum alloy processed at a high temperature of 200° C. or above, an oxide layer is formed. In this oxide layer, a number of cracks are present, and the cracks cause a lowering of anticorrosion. The thickness thereof is also non-uniform. When processed at a high temperature as in the present invention, the material way undergo deformation and oxide layer formation at the same time, so that the layer becomes partly thick and partly thin. Accordingly, in order to obtain a good anticorrosion, a uniform and dense coating must be formed so that the disadvantages of the natural oxide layer can be compensated.
In the present invention, an aluminum alloy containing, e.g., 2% or more of Mg is used. The Mg tends to segregate on the surface of aluminum, and any presence of an oxide layer on the surface further promotes the tendency of segregation. The Mg having segregated on the surface forms magnesium oxide by its reaction with oxygen in the atmosphere. The magnesium oxide is alkaline, and adversely affect the anticorrosion, adhesion of coatings and surface appearance. Hence, it is necessary to eliminate the influence of Mg on the surface or in the vicinity of the surface.